Correlated Grading

[jolyonb]

When we have a ListGrader that contains multiple inputs (lowest-level of nesting), the grading for an input should be able to depend on the content of another input. A simple example would be "enter any number here", then "enter its square here". Because the first entry can accept basically anything, that would require a custom comparer. The second entry would also require a custom comparer. The easiest implementation of this would be to allow a custom comparer to have a special parameter that evaluates another input. We'd just need to be careful so as to have no circular references!

[jolyonb]

Resolved by #80.